The refill tube should be secured to the overflow in some manner, if by no other means than a tight fit in the opening. It can come out because it is not secured, or it could be because too much water is exiting the tube and creating a "jet" effect which pushes it out of the overflow. From here it is not possible to tell if the valve is plugged up and sending too much water to the overflow, or if there is some other problem unique to your installation.

: I hope someone can help. I would like to fix my toilet but I am inexperienced and am not quite sure what has caused the problem. The toilet flushes, it empties the bowl and the tank refills. The bowl only refills at a fraction of what it used to. All of a sudden the refill tube is not staying in the overflow tube as it should. In fact, if I lift the tank lid as it flushes, the tube is spewing water all over the tank and sometimes me. I can place the refill tube back in the overflow tube and it will be good for one flush (maybe two) after that the tube has been pushed out again and the bowl does not completely refill. Any quick fixes? Thanks!